fix(android): dismiss overlays on system back instead of navigating away

[eleboucher]

Description

Add useDismissOnBack(requestClose, enabled) hook to androidBack utils and wire it into full-screen and interaction-blocking overlays. On Android the system back action (edge-swipe and 3-button nav) was passing through these overlays and navigating the underlying route, leaving the overlay open.

Wired overlays:

• Modal500: Room Settings, Space Settings, Settings shallow route
• MobileSwipeDownModal: message context menu, header menu, nav sidebar menu
• ImageViewer (refactored from inline handler)
• GlobalModalManager: Delete, Report, Source, Reactions, EditHistory, ReadReceipts (gated, excludes Forward and MobileOptions)
• MessageForward: forward picker
• IncomingCallModal (gated by open state)
• DeviceVerification
• Emoji boards in RoomInput, Reactions, MessageEditor (gated, the latter via a MobileEmojiOverlay wrapper since its state comes from a UseStateProvider render-prop)
• FileContent text and PDF viewers (gated)

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).